You seem to have two main issues. Firstly, you need to call the Close method on the WordprocessingDocument in order for some of the document parts to get saved. The cleanest way to do that is to use a using statement around the WordprocessingDocument. This will cause the Close method to get called for you. Secondly, you need to Seek to the beginning of the stream otherwise you'll get an empty result.

You also have the incorrect file extension and content type for an OpenXml file but that won't typically cause you the problem you are seeing.